6X8 sub?

I have a f-250 crew cab so the rear speakers are behind the seat and get most of the highs filtered out. I have an older JBL 300.4 amp and wondered if there was such thing as 6X8 subs I could mount in the back. Getting ready to replace the OEM HU with a Pioneer and put Kappas up front. Don't have room behind the seat due to the large amp and back window motor. Don't need a lot of thump just a little low end filler.